The present invention relates generally to circuits for driving inductive loads, and more particularly to a high speed CRT deflection yoke driver.
The switching of current through an inductive load can be achieved through the use of a transistor whose primary current path is connected in series with the current path through the inductive load, and by then switching the transistor between saturation and cut off. When high speed switching of the inductive load is required, however, this approach is not entirely satisfactory. The transistor storage time resulting from saturation of the transistor during its "on" state significantly limits the switching speed of the transistor. To circumvent this problem, it was the practice in the past to include a clamping circuit across the transistor. This clamping circuit prevented the collector-emitter potential from dropping below a certain limit, thus preventing the transistor from dropping into saturation. In order to prevent the transistor from drawing unlimited current from the clamping circuit, however, it was then necessary to connect the transistor in a current source (or sink) configuration. The current through the transistor was therefore limited to a preset level when it was in the "on" state. The resulting driver circuit was capable of providing much more rapid switching speeds than had been available when the transistor had been utilized without these refinements.
Certain cathode ray tube (CRT) display systems require high speed inductive switching of this nature. In some advertising layout systems using video display terminals, for example, the electron beam is moved to a particular position on the fact of the CRT, and then a small area of the screen, sometimes referred to as a character field, is scanned in a conventional raster scanning technique. A particular character or other design is thus displayed within that character field. An additional complication arises in these applications by the need to vary the size of the character field in order to display characters having different point sizes. In terms of the deflection circuitry, this means that the deflection driver circuit must not only be capable of operating at high speed, but must also be capable of operating at different levels of deflection current, depending upon the size of the character which is to be displayed.